1. Field of the Invention
The present invention relates to an optical fiber typically used for optical wiring inside an apparatus.
2. Description of the Related Art
Schemes used for signal transmission inside an apparatus can be two types: electric transmission scheme and optical interconnection scheme. With the recent increase of the speed of CPU clock frequencies, occurrence of cross-talk, which is caused by high density wiring, is a problem for the electric transmission scheme. Therefore, application of a waveform shaping technique, etc., is necessary. As a result, when the electric scheme is employed as the signal transmission scheme in an apparatus, it is known that a transmission distance of about 1 m and a transmission speed of about 10 Gbps are the limits for the transmission.
On the other hand, the optical interconnection scheme enables execution of transmission over a much broader band compared to the electric transmission scheme and enables configuration of a signal transmission system that uses small-size low-power-consumption optical components. Therefore, the optical interconnection scheme is drawing attention as an in-apparatus signal transmission technique that is to replace the electric transmission scheme.
A scheme that employs an optical fiber is also drawing attention as one of optical transmission units employing the optical interconnection scheme. All the optical components used in an apparatus are desired to be able to be accommodated in a space as small as possible. Therefore, an optical fiber to be used for the optical interconnection scheme is desired to enable flexible wiring and to have a small connection loss between the optical fibers or between the optical fiber and a light source, etc.
On the other hand, as to the light source, Vertical-Cavity Surface-Emitting Laser, hereinafter, “VCSEL” that operates at the direct modulation from 2.5 Gbps to 10 Gbps is drawing attention as an access-based, an Ethernet (a registered trademark), and a fiber channel un-cooled light sources. VCSEL has the following features compared to an edge-emitting laser such as a distributed-feedback (DFB) laser.
(a) A laser light beam is output perpendicularly to the substrate surface.
(b) Can be easily configured as a multi-channel array.
(c) Can operate at a low threshold value and with small power consumption.
(d) The emitting surface has a high reflectivity and it is insensitive to the returned light (isolator-free).
(e) Shape of the emitted light beam is circular and the light beam has a high coupling factor with a fiber (lens-free).
Because the components such as an isolator and a lens can be omitted as explained above, the number of components can be reduced. In other words, the VCSEL is a device that can facilitate reduction of the cost as a module. A VCSEL for a 850-nm band of wavelength and using a GaAs/AlGaAs quantum, etc., as an active layer is already prevailing widely as a de facto standard of short-distance communication laser elements. A typical optical fiber used in this case can be a silica-based graded-index optical fiber that is a type of multi-mode fiber (hereinafter, “MMF”).
An MMF is characterized in that it has a core diameter that is as large as that of a single-mode fiber (hereinafter, “SMF”), and a large numerical aperture. Therefore, high precision is not necessary for a connection between optical components such as between optical fibers or between an optical fiber and a light source, etc. Therefore, easy connection is enabled.
Recently, aiming at executing higher-speed communication, considering application of an SMF that has a smaller loss and a wider band compared to an MMF is started. The VCSEL that has an oscillation wavelength in a 1.3 μm band (1,300±50 nm) that is a small-loss band for silica-based optical fibers, is drawing attention as a light source used in this case, and research and development of the VCSEL are actively carried out.
However, a standard SMF provided in ITU-T (International Telecommunication Union Telecommunication Standard Sector) G.652 can not cope with the above communication because a large bending loss occurs when this SMF is accommodated in an apparatus.
As to an SMF having lesser bending loss, an optical fiber including a portion that has a lower refractive index than that of a cladding in the cladding portion of the standard SMF and, therefore, having a trench-shaped refractive index profile has been reported as an optical fiber preferably used for FTTH (Fiber To The Home) (for example, Non-Patent Document 1). However, this fiber is incomplete to be used in an optical interconnection system.
Non-Patent Document 1: Masataka Ikeda, Shoichiro Matsuo, and Kuniharu Himeno, “Low Bending Loss Optical Fiber with Reduced Splice Loss”, Technical Report of Institute of Electronics, Information and Communication Engineers, OCS2003-43, OFT2003-25 (2003-8)
As above, as to an optical fiber to be used in an optical interconnection system, it is required to realize an optical fiber for which both of the bending loss thereof and the connection loss thereof are reduced, that enables high-speed optical transmission, and that is suitable for easily constructing the optical interconnection system.
A standard SMF can not cope with any optical interconnection system because a large amount of bending loss occurs to the SMF. Therefore, it is necessary to reduce the bending loss of an optical fiber. However, even SMFs for which the bending loss has been reduced and that are preferably used for FTTH, including the above optical fiber described in Non-Patent Document 1, are not suitable for use in an optical interconnection system.